Communication devices must comply with various emission requirements and other spectrum constraints defined by a number of regulatory bodies, such as the Federal Communications Commission (FCC) in the United States, or communication standards, such as the IS-95 Base Station Emission Requirements, defined in TIA/EIA-95-B, “Mobile Station—Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems,” Section 7—Requirements for Base Station CDMA Operation. Adjacent Channel Power Ratio (ACPR), for example, is one such constraint that is generally defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted frequency channel. In other words, while a wireless communication device may be configured to transmit over a specific frequency channel at any given time, it is difficult to prevent the energy transmitted by the device from spreading over into adjacent channels. If too much energy resides in the adjacent channels, then the energy can interfere with devices operating on the adjacent channels.
One source of ACPR is the inherent non-linearity found in various communication device components, including RF amplifiers. Currently, two methods are employed to compensate for non-linearities in RF amplifiers. Feedback methods continuously track the output signal of an amplifier and correct for non-linearities using a negative feedback loop. Predistortion methods predistort the magnitude and phase of the input signal so as to cancel distortion introduced by the amplifier, and thereby provide a linear output signal.
Generally, predistortion methods inject a predistortion signal into the input signal path of the RF amplifier. The predistortion signal is approximately equal and opposite to the expected distortion at the amplifier output, in order to cancel the distortion of the RF amplifier. The distortion of an amplifier varies with the temperature and age of the amplifier. Thus, a number of adaptive predistortion techniques have been proposed or suggested to address the varying temperature and aging effects. Adaptive predistortion methods attempt to continuously minimize the distortion in the amplifier output using an error signal.
While existing predistorters effectively compensate for non-linearities in RF amplifiers, they suffer from a number of limitations, which if overcome, could further improve the efficiency of RF amplifiers. In particular, in order to generate the power levels required for evolving communication systems, without excessive heat and within the relevant spectrum constraints, a need exists for improved predistortion systems that allow amplifiers to be more efficient and generate higher output power levels.